The present invention relates to brake control systems for railroad freight trains and in particular, to freight trains employing local electro-pneumatic control of brake pipe pressure for controlling operation of the individual car brakes.
Present day freight trains have a brake pipe that runs through each car and is coupled therebetween so as to extend continuously the length of the train. The brake pipe is charged with compressed air typically at the head end by a compressor on the locomotive. The compressed air not only provides the pneumatic brake force at the respective cars, but also serves as a communication link via which the car's brakes are controlled from the locomotive by increasing and decreasing the brake pipe pressure.
Due to the length of modern day freight trains, considerable time is required for the pneumatic control signals to propagate from the front to the rear cars of the train. This can present difficulty in controlling the train, particularly on long trains operating over undulating terrain. Accordingly, electrical means have been proposed for near-instantaneously transmitting the brake pipe pressure control signals to at least one car of the train, such as the last car, or to several cars randomly situated within the train consist, or to all of the cars, depending upon the train make-up. Near-instantaneous remote control of the car brakes may be accomplished either by means of radio signals or by a train line wire, for example. Such an arrangement has the potential of providing greatly improved train performance, due to faster brake applications realized by such remote brake pipe pressure control.
However, due to the natural, fully charged pressure gradient that can exist in a train brake pipe, undesired release of the brakes on cars located toward the rear of the train can occur if remote brake pipe pressure reductions are not carefully controlled. Such pressure gradient exists as a result of the inevitable leakage of compressed air at the hose couplings that connect the brake pipe between cars or at other sources, and due to brake pipe flow resistance encountered in maintaining this leakage by means of the typical operator's brake valve on the locomotive. By electrically controlling a brake pipe pressure reduction to initiate a brake application, it will be appreciated that the brake pipe pressure will be reduced by a corresponding amount at each car equipped with electro-pneumatic control of brake pipe pressure. Because of the natural pressure gradient and the pressure maintaining function of the locomotive brake valve, any pressure reduction that exceeds the natural pressure gradient at the respective cars can subsequently cause that car's brakes to be inadvertently released, as the rearwardly disposed cars brake pipe pressure rises back to a pressure consistent with the natural train pressure gradient at the reduced brake pipe pressure. This phenomenon can be explained by way of the graph of FIG. 1A.
In FIG. 1A, curve A represents the brake pipe pressure when charged to 90 psi at the lead locomotive. The natural gradient of this curve A shows that at car 100, the brake pipe pressure is only 80 psi, when fully charged, a 10 psi gradient from car 1 to car 100. Curve B represents a temporary false brake pipe pressure gradient that exists immediately following an electrically initiated reduction of 15 psi brake pipe pressure throughout the train. Since different pressure heads produce different gradients, it will be appreciated that the true natural gradient of the reduced brake pipe pressure, curve C, differs from that of curve A. Consequently, the brake pipe pressure due to the maintaining feature of the locomotive brake valve will rise toward the rear of the train as the reduced brake pipe pressure seeks its new natural gradient. As is well understood by those in the railroad brake art, an increase in brake pipe pressure of approximately 2 psi can cause a car control valve device to release the car brakes. Such a rise in brake pipe pressure following an electrically initiated brake application, as above explained, can therefore have the undesirable effect of inadvertently releasing the brakes on those cars toward the rear of the train where such rise in brake pipe pressure is most pronounced.
Electric control of brake pipe pressure, as above discussed, can also cause inadvertent release of train brakes, particularly on rear-end cars, for a somewhat different, but related reason. When making an electric brake application following release of a previous application, for example, such re-application may be initiated prior to the brake pipe pressure of the initial application being fully recharged. Such a possibility exists since brake pipe pressure is charged from the locomotive at the head of the train and therefore passes serially through each car, so that charging of the rear end cars is accordingly delayed. This delayed brake pipe pressure build-up on cars situated near the rear end of the train gives rise to what is typically referred to as a temporary false (less than fully charged) pressure gradient in the brake pipe. In that this false pressure gradient gradually decreases, in achieving a natural pressure gradient consistent with the locomotive brake pipe pressure, the brake pipe pressure increase on cars located near the rear of the train may be sufficient to cause an inadvertent and undesirable release of the brakes on these cars.
In FIG. 1B, curve W represents the brake pipe pressure when charged to 90 psi corresponding to curve A in FIG. 1A. The natural gradient of this curve W shows that at car 100, the brake pipe pressure is 80 psi, a 10 psi gradient from car 1 to car 100. Curve X represents the natural brake pipe pressure gradient between car 1 and 100 following a brake pipe pressure reduction from 90 psi to 75 psi at the locomotive, when a brake application is made. This curve corresponds to curve C in FIG. 1A. Curve Y represents the brake pipe pressure in the course of being increased to effect a brake release.
As explained in the foregoing, the apparent delay in recharging the brake pipe produces a false pressure gradient, which depending on train length, degree of leakage, etc., can be quite steep. In the example of FIG. 1B, this false gradient represented by curve Y is assumed to be such that the pressure at car 100 is 75 psi, when a re-application of the train brakes is initiated. Curve Z represents a reduced brake pipe pressure corresponding to the desired brake effort when re-applying the brakes. Assuming a 15 psi reduction throughout the train, the brake pipe pressure at the locomotive is reduced from 90 psi to 75 psi, while the brake pipe pressure at car 100 is reduced from 75 psi to 60 psi. Curve Z thus reflects the false gradient effective at the time the brake re-application is completed.
It will now be appreciated that the brake pipe pressure will gradually rise from front to back in seeking the natural pressure gradient consistent with the reapplication brake pipe pressure at the locomotive. If this re-application brake pipe pressure is assumed to have a natural gradient, as represented by curve V, such that the brake pipe pressure at car 100 is 67.7 psi, the subsequent rise in brake pipe pressure from 60 psi to 67.7 psi will be considerably more than enough to effect a release of the brakes on car 100; and in addition, this rise in brake pipe pressure will progress forward from car 100 to release the brake on those cars where the difference between the reapplication brake pipe pressure of curve Z and the natural gradient pressure of curve V is greater than approximately 2 psi, the pressure differential at which brake release generally occurs. Of course, the greater the false pressure gradient at the time the re-application is initiated, the greater will be the danger of reducing the brake pipe pressure near the rear of the train below the pressure representing the natural gradient following the brake application.
It is therefore highly desirable to cause the brake pipe pressure throughout a long train to be reduced to a pressure as close as possible to the new natural gradient when a service brake pipe reduction is made. With electronic brake signal and control, this maximizes the rapid effectiveness of braking throughout the train. If the brake pipe pressure is reduced substantially less on the rear of the train than it is at the front of the train electrically, the remaining pressure reduction down to the new natural gradient will occur very slowly through pressure exhaust at the locomotive. On the other hand, reducing the brake pipe pressure below the natural gradient creates the danger of an undesired brake release.